Sorry if I'm teaching you to suck eggs, but these are old gearbox's, are you changing gear too quickly? Clutch right to the floor and feed it in letting the synchro do the work. If it still crunches let us know.

Only solution is rebuild time, that said though when i first got my mini it would crunch going to 3rd and it didn't get any worse for the next 40k i did in it until it was time to remove the engine for a rebuild due to excessive oil consumption! You can probably get around the issue with a bit of heel and toe or double declutching as mentioned above

If it is allowed to continue to 'crunch' when changing gear, the gearbox will wear further.

There are two options:

1. Re-build the gearbox

2. Double de-clutch when changing gear (not heel & toe, that is something entirely different). The gearbox in the early Minis had syncromesh which didn't last long and we all double de-clutched back then.